Compressed gas storage unit

ABSTRACT

Embodiments of the present invention relate to compressed gas storage units, which in certain applications may be employed in conjunction with energy storage systems. Some embodiments may comprise one or more blow-molded polymer shells, formed for example from polyethylene terephthalate (PET) or ultra-high molecular weight polyethylene (UHMWPE). Embodiments of compressed gas storage units may be composite in nature, for example comprising carbon fiber filament(s) wound with a resin over a liner. A compressed gas storage unit may further include a heat exchanger element comprising a heat pipe or apparatus configured to introduce liquid directly into the storage unit for heat exchange with the compressed gas present therein.

CROSS-REFERENCE TO RELATED APPLICATION

The instant nonprovisional patent application claims priority to U.S.Provisional Patent Application No. 61/589,254, filed Jan. 20, 2012 andincorporated by reference in its entirety herein for all purposes.

BACKGROUND

Compressed air is capable of storing energy at densities comparable tolead-acid batteries. Moreover, compressed gas does not involve issuesassociated with a battery such as limited lifetime, materialsavailability, or environmental friendliness. Thus, there is a need inthe art for apparatuses and methods allowing the creation of storageunits for compressed gas.

SUMMARY

Embodiments of the present invention relate to compressed gas storageunits, which in certain applications may be employed in conjunction withenergy storage systems. Some embodiments may comprise one or moreblow-molded polymer shells, formed for example from polyethyleneterephthalate (PET) or ultra-high molecular weight polyethylene(UHMWPE). Embodiments of compressed gas storage units may be compositein nature, such as comprising high strength (e.g. carbon) fiber wound inthe presence of a resin over a liner. The compressed gas storage unitmay further include a heat exchanger element such as a heat pipe, finnedtube, or apparatus introducing liquid directly into the storage unit forheat exchange with the compressed gas present therein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a simplified cross-sectional view of a storage unit forcompressed gas according to an embodiment.

FIGS. 1A-B show highly simplified views of a process of fabricating acompressed gas storage unit utilizing blow molding.

FIG. 2 shows an embodiment of a multi-shell compressed gas storage unit.

FIG. 3 shows another embodiment of a multi-shell compressed gas storageunit.

FIG. 4A shows a process utilizing a plurality of molds to form amulti-shell compressed gas storage unit according to an embodiment.

FIG. 4AA shows another embodiment of a multi-shell compressed gasstorage unit.

FIG. 4B shows a process utilizing a plurality of molds to form amulti-shell compressed gas storage unit according to an embodiment.

FIG. 4BA shows another embodiment of a multi-shell compressed gasstorage unit.

FIGS. 5A-CB show the use of blow molding to fabricate an embodiment of acompressed gas storage unit having a flange feature according to anembodiment.

FIG. 5D shows an alternative embodiment of a mold for forming acompressed gas storage unit having a flange feature.

FIG. 5E shows an embodiment of a multi-shell compressed gas storage unithaving a flange feature.

FIG. 6 is a simplified view of an embodiment of a tool which may be usedto fabricate a composite filament-wound compressed gas storage unit.

FIG. 7 shows an embodiment of a liner having a collapsible stiffenerdisposed therein during a filament winding fabrication process.

FIG. 8 shows application of a figure of merit to a number of differentfiber types.

FIG. 9 is a table showing characteristics of the fiber types of FIG. 8.

FIG. 10 shows an embodiment of a compressed gas storage module.

FIG. 11 shows an embodiment of a compressed gas storage unit including aheat exchanger.

FIGS. 11AA-AB show perspective and cross-sectional views of anotherembodiment of a compressed gas storage unit including an internal heatexchange element.

FIG. 12 shows an alternative embodiment of a compressed gas storage unitincluding a heat exchanger.

FIG. 13 shows an embodiment of a compressed gas storage unit havingpolar end pieces and oriented along an axis.

FIG. 14 shows an embodiment of a compressed gas storage unit having anexpandable heat exchanger present therein.

FIGS. 15-18 show embodiments of compressed gas storage units employing aliner for heat exchange.

FIGS. 19-25 show various views of a compressed gas storage unitaccording to an embodiment.

FIG. 26 plots a cost per kWhr for compressed gas storage unitsfabricated from various materials.

FIG. 27 shows a simplified view of an embodiment of an energy storagesystem.

FIG. 28 shows a simplified view of an alternative embodiment of anenergy storage system.

FIG. 28A shows various basic operational modes of the system of FIG. 28.

FIGS. 28BA-BF show simplified views of the gas flow paths in variousoperational modes of the system of FIG. 28.

FIG. 29A shows a side elevational view of a configuration utilizing avertical folded configuration for pressure vessels.

FIG. 29B shows a plan view of a configuration utilizing a serpentinehorizontal folded configuration for pressure vessels.

FIG. 30 shows a simplified view of an embodiment of a configuration thatmay be used in pneumatic applications.

DESCRIPTION

U.S. Patent Publication No. 2011/0115223 (“the '223 Publication”)describing an energy storage and recovery system employing compressedgas as an energy storage medium, is hereby incorporated by reference inits entirety for all purposes.

FIG. 1 shows a simplified cross-sectional view of a storage unit 100 forcompressed gas according to an embodiment. Storage unit 100 comprisesshell 101 and end piece 103 which together enclose an interior space105. In certain embodiments, the end piece may comprise a boss. The endpiece defines a port 107 through which gas can enter or leave thestorage unit. FIG. 1 is highly simplified and drawn for purposes ofillustration, and thus the relative dimensions may not reflect actualvalues.

In this particular embodiment, a blow molding technique is employed toform the storage unit 100. Specifically, as shown in FIG. 1A a parison102 comprising hollow or concave plastic material, is inserted into amold 104 defining an internal cavity 106. The parison 102 may compriseone or more materials capable of stretching in response to an appliedpressure.

For example, the parison may typically comprise a polymer material,which may be thermoplastic, comprising relatively long polymer chains.Stretching of the polymer chain in response to an applied pressure,desirably transmits force along a direction of the chains, straighteningthem out and thereby allowing resulting internal tension to be taken upby the strong covalent bonds extant between elements within the chains.

Moreover, the Poisson effect may be observed in a blow molding process.Specifically, a stretching may occur in multiple directions on thesurface of the vessel. As the blow molded material is stretched in aplane it becomes thinner, thereby further constraining polymer chains tolie within that plane.

An example of a polymer which may be used in blow molding ispolyethylene terephthalate (PET or PETE). Another example of a polymerwhich may be suited for the formation of compressed gas storage unitsvia blow molding techniques, is ultra-high molecular weight polyethylene(UHMWPE). Such UHMWPE materials may be characterized by chains havingrelatively high molecular weight (e.g. >1×10⁶).

As shown in FIG. 1B, a gas 110 is then introduced under pressure intothe parison 102 disposed within the cavity 106 of the mold. The forceresulting from the pressure of the blown gas, causes the material of theparison to expand against the internal walls of the mold defining thechamber, thereby forming the shell 101.

In later steps, the mold is removed and in certain embodiments may bereused. The end piece 103 furnishing the port 107 to allow the passageof gas into the shell, is then provided.

The blow molding process shown in FIGS. 1A-B is highly simplified.Additional steps may be performed in various embodiments.

For example, certain blow molding processes may involve the heatingand/or cooling of individual elements at the same or different times.For example, the parison and/or the mold may be heated or cooled atvarious points in the process.

Other types of processes which can be performed at various stages ofblow molding include spinning, injection, extrusion, curing, trimming,and/or milling. Various types of additional materials, for example inthe liquid or gas phase, can also be introduced during the process inorder to achieve the desired structure.

The following equation provides a Figure of Merit (FOM) as a measure toassess relative costs of blow molded compressed gas storage units madefrom various materials, with a lower FOM representing a lower costsolution:

${FOM} = {\left( \frac{SF}{{\overset{\_}{\sigma}}_{p}} \right)\left\lbrack {\$_{p} + {{\$_{r}\left( \frac{1 - v_{p}}{v_{p}} \right)}\left( \frac{\rho_{r}}{\rho_{p}} \right)}} \right\rbrack}$

SF=Safety Factor

σ _(p)=Specific Strength of polymer after blow molding$_(p)=Unit Price of polymer$_(r)=Unit Price of Resinν_(p)=Polymer Volume Fractionρ_(r)=Resin Densityρ_(p)=Polymer Density

While the above figures illustrate an example wherein blow moldingtechniques are employed to fabricate a shell having a concave shape,various types of shapes could be formed depending upon the profile ofthe interior cavity of the mold. Examples of alternative shapes includebut are not limited to cylindrical and spherical. The latter shape mayoffer a benefit by consuming a least amount of shell material per volumeof gas enclosed.

Moreover, while FIGS. 1A-B show a particular example of fabricating acompressed gas storage unit having a single shell by injection molding,embodiments of the present invention are not limited to having this orany number of shells. Thus particular embodiments could comprise aplurality of blow molded shells arranged in a nested configuration, forexample as is shown at least in the particular embodiments of FIGS.2-4BA.

There is no theoretical limit to a number of shells which may beemployed to form a blow molded compressed gas storage unit. Examples ofa number of blow molded shells which may comprise a compressed gasstorage unit, include but are not limited to 1, 2, 3, 4, 5, 6, 7, 8, 9,10, 15, 20, 50, 75, and 100 or an even greater number.

Embodiments of compressed gas storage units formed by blow molding, andparticularly those comprising multiple shells, may be expected tocontain gas at high pressures. Examples of internal gas pressures thatthe blow molded compressed gas storage units may be expected towithstand, including but are not limited to 100 psig, 200 psig, 250psig, 500 psig, 1000 psig, 1500 psig, 2000 psig, 2500 psig, 3000 psig,3500 psig, 4000 psig, 4500 psig, 5000 psig, 5500 psig, and 6000 psig.Examples of internal volumes which may be defined within compressed gasstorage units formed by blow molding, include but are not limited to0.002 m³, 0.005 m³, 0.01 m³, 0.025 m³, 0.05 m³, 0.1 m³, 0.5 m³, 1 m³, 2m³, 2.5 m³, 3 m³, 4 m³, 5 m³, 10 m³.

Some embodiments comprising multiple shells could be formed by asuccession of blow molding steps performed within the same mold. Thatis, blow molding of a first parison to form a first shell, could befollowed by in turn by insertion of a second parison and blow molding toform the second shell against the first shell.

According to certain embodiments, the parison could include one or moreraised features such as ribs. Upon stretching of the parison within theinternal cavity of the mold, these ribs could contact the mold wall toserve as stand offs defining spaces that are enclosed within the wall ofthe storage unit. Such spaces could remain open, or could later befilled with other materials such as gases or liquids in order to achievea desired effect. For example, in certain embodiments the interveningmaterial could comprise a heat exchange fluid. The resulting shape ofsuch a multi-shell structure is shown simplified in cross-section inFIG. 2, with space 203 lying between shells 202 and 204 comprising theunit 206.

Moreover, in certain embodiments intermediate steps could be performedbetween blow molding steps to introduce in-situ, materials interveningbetween successive shells. That is, the first molded shell could beexposed to a material imparting desired characteristics, prior to blowmolding of the second shell. The resulting shape of such a structure 300comprising multiple blow molded shells 302 and 304 including interveninglayer 306, is shown in simplified cross-section in FIG. 3.

Examples of such intermediate materials may include but are not limitedto adhesives, epoxies, surfactants, sealants, thermal insulators,thermal conductors, corrosion-resistant layers, diffusion barriers,foaming agents, materials affecting optical properties, water, air,and/or silicone rubber.

While FIGS. 2 and 3 show embodiments of compressed gas storage unitscomprising multiple shells successively blow molded in-situ within asame mold, this is not required. Alternative embodiments of compressedgas storage units comprising multiple shells could comprise a pluralityof shells formed by blow molding processes, followed by assembly of theplurality of shells in a nested configuration.

One such embodiment is shown in FIG. 4A, where mold(s) 402 of the samesize are used to blow mold a plurality of identical shells 404 having aconical shape. Those shells are then combined to form the assembledshell 406, and an end piece 407 is added to create the compressed gasstorage unit 408.

As shown in the alternative embodiment of FIG. 4AB, in certainembodiments the assembled shell 406 may then be coupled with anotherassembled shell 410 to form the resulting storage unit 412. The shells406 and 408 may be maintained in contact with one another using bondingelement(s), for example a penetrating element such as a bolt as shown,or alternatively using a bonding element such as an adhesive, or aclamp.

Another embodiment is shown in FIG. 4B, where mold(s) 450 of differentsizes are used to blow mold a plurality of shells 454 of different sizedhemispheres. Those shells are then combined to form the assembled shell456, with an end piece 457 attached thereto by a clamp 459 to create thecompressed gas storage unit 458.

As shown in the alternative embodiment of FIG. 4AB, in certainembodiments the assembled shell 456 may then be coupled with anotherassembled shell 460 to form the resulting storage unit 462. The shells456 and 460 may be maintained in contact with one another using bondingelement(s), for example a clamp as shown, or alternatively others suchas an adhesive, or a bolt.

Use of a blow molding according to embodiments may allow the fabricationof a compressed gas storage unit having desired shapes or features. Forexample, FIGS. 5A-CB show the use of blow molding to fabricate anembodiment of a compressed gas storage unit having a flange feature. Inparticular, following insertion of the parison 500 into mold 502featuring internal projections 504 (FIG. 5A), in FIG. 5B the parison isexpanded by blow molding. The resulting flange 506 is produced by thematerial of the parison conforming to the projection in response to theapplied pressure.

As shown in FIGS. 5B-CA, subsequent cutting along the dashed linesyields a flange insert 508 of the same material as the mold (which maybe metal). This flange insert 508 may serve to further strengthen theflange feature and the resulting compressed gas storage unit.

FIG. 5CB shows an end view after the subsequent drilling through theflange including the insert. The resulting fitting allows joining of theblow molded structure to another, for example an end cap or even anothercompressed gas storage unit to form a modular structure.

FIGS. 5A-CB show an embodiment comprising a compressed gas storage unithaving a cylindrical shape. A possible benefit of having such acylindrical tank shape is that the parison could be a simple straighttube. Extruded tubing may be available in sufficient length for largetanks, and cost less than an injection molded parison of the appropriatesize.

FIG. 5D shows a simplified cross-sectional view of a mold according toanother embodiment comprising a spherical shape. In this embodiment, theflange portion 580 of the mold 582 is set off from the interior cavity,such that channels 584 for penetrating bolts do not enter the cavity.This can possibly strengthen the resulting blow molded spherical chamberunder high pressure, and also eliminate a need to form an airtight sealwith the interior through the bolt holes.

A multi-shell structure corresponding to that of FIGS. 5A-D could befabricated by following the cutting step with the insertion of epoxyfollowed by an additional sequence of parison insertion/blowmolding/cutting steps. An example of this is shown in simplified form inFIG. 5E.

1. A compressed gas storage unit comprising:

a first shell formed by blow molding a first polymer parison within amold; and

a second shell disposed within the first shell and formed by blowmolding a second polymer parison.

2. A compressed gas storage unit as in clause 1 wherein the second shellis formed by blow molding the second polymer parison within the firstshell.

3. A compressed gas storage unit as in any of the previous clauseswherein the second shell is inserted into the first shell.

4. A compressed gas storage unit apparatus as in any of the previousclauses wherein the second shell is in contact with the first shell.

5. A compressed gas storage unit apparatus as in any of the previousclauses further comprising an intervening material between the secondshell and the first shell.

6. A compressed gas storage unit apparatus as in any of the previousclauses wherein the intervening material comprises an epoxy.

7. A compressed gas storage unit apparatus as in any of the previousclauses wherein the first shell further comprises a flange feature.

8. A compressed gas storage unit apparatus as in any of the previousclauses wherein the first shell comprises ultra high molecular weightpolyethylene (UHMWPE).

9. A compressed gas storage unit apparatus as in any of the previousclauses wherein the first shell comprises polyethylene terephthalate(PET).

10. A compressed gas storage unit apparatus as in any of the previousclauses wherein the first shell has a shape comprising a cylinder, asphere, a hemisphere, or a cone.

11. A compressed gas storage unit apparatus as in any of the previousclauses further comprising a wound filament in contact with the firstshell.

12. A method of fabricating a compressed gas storage unit, the methodcomprising:

disposing a first polymer parison within a mold defining a cavity;

blow molding the first polymer parison within the mold to create a firstshell conforming to a shape of the cavity; and

providing a second shell within the first shell.

13. A method as in clause 12 wherein providing the second shellcomprises:

disposing a second polymer parison within the first shell; and

blow molding the second polymer parison within the first shell to definethe second shell.

14. A method as in any of the previous clauses wherein providing thesecond shell comprises:

blow molding the second shell from a second polymer parison within asecond mold; and

inserting the second shell within the first shell.

15. A method as in any of the previous clauses further comprisingintroducing a material within the first shell prior to providing thesecond shell.

16. A method as in any of the previous clauses wherein the materialcomprises an epoxy.

17. A method as in any of the previous clauses wherein the shape of thecavity is cylindrical.

18. A method as in any of the previous clauses wherein the shape of thecavity is spherical.

19. A method as in any of the previous clauses wherein the first polymerparison comprises ultra high molecular weight polyethylene (UHMWPE) orpolyethylene terephthalate (PET).

20. A method as in any of the previous clauses wherein the first shellcomprises polyethylene terephthalate (PET).

21. A method as in any of the previous clauses further comprisingwinding a filament around the first shell.

While the above figures have described the formation of one or moreelements of a compressed gas storage unit by blow molding, this is notrequired. Other embodiments could employ alternative techniques andremain within the scope of the present invention.

For example, certain embodiments may employ the winding of filamentsabout a liner to fabricate a compressed gas storage unit out ofcomposite materials. One candidate for such a composite material iscarbon fiber.

Carbon fiber composites were introduced in the 1960s. As a structuralmaterial, benefits of carbon fiber composites include high specificstrength (strength per unit weight) and specific modulus (stiffness perunit weight) when compared to other classes of materials.

Carbon fiber was initially a relatively expensive product, and saw itsfirst usages in aerospace and defense applications. Now, costs are morecompetitive and usage is fairly widespread. Over the past decade, carbonfiber has found its way into many consumer products as well,particularly sporting goods such as bicycle frames, golf club shafts,and skis.

The mechanical properties of carbon fibers can vary. Strength values forcarbon fiber range from about 500 to 725 ksi. Fibers are also commonlyclassified by their elastic modulus (or stiffness).

Standard modulus fibers are the cheapest fibers available today, and seewidespread use in general aviation and consumer applications. Standardmodulus fibers have an elastic modulus of approximately 33 Msi.

Intermediate modulus fibers are a more expensive fiber that may be usedfor stiffness critical structures, including military aircraft, launchvehicle structures, rocket motor casings, and primary loadbearingstructure on commercial aircraft. Intermediate modulus fibers in generalhave an elastic modulus of approximately 42 Msi.

High modulus carbon fibers are more expensive fibers that are used inspacecraft applications or those requiring high dimensional stability.High modulus fibers in general have an elastic modulus of greater than50 Msi.

The precursor is the raw material from which the carbon fiber isproduced. Approximately 50% of the cost of carbon fiber may beattributable to the precursor used to synthesize the fiber, andprocessing of that precursor.

Currently much carbon fiber is made from one precursormaterial—polyacrylonitrile (PAN). PAN is petroleum based, and thereforeits price fluctuates with crude oil prices.

Other precursor materials include pitch, novoloid (a phenolic), andrayon. These are more expensive to process, and the fiber itself mayhave a low modulus and less favorable mechanical properties. Fibers madefrom pitch, novoloid, and rayon may be used for high temperature andablative aerospace applications.

Over the past decade, significant research activity has focused on theidentification and processing of new, low cost precursors for carbonfiber. The range of potential precursor materials currently beingevaluated includes cellulose, textile-grade PAN, and polyolefin-basedprecursors.

Since a large proportion of the cost of carbon fibers is associated withthe precursor material, an opportunity exists for a low cost carbonfiber based on alternative precursors, especially those that come fromrenewable resources. Lignin is a polymer that occurs in the cell wallsof plants. Lignin is a by-product of bio-diesel production: a givenquantity of bio-mass produces more lignin than fuel.

Certain bio-fuel manufacturing processes result in unadulterated ligninbeing produced as a by-product. The lignin is in the form of dry flakeswith low chloride content. The use of high quality Lignin precursors mayallow the synthesis of carbon fibers having a stiffness of approximately25 Msi, and a strength target of 250 ksi.

Standard modulus carbon fiber is synthesized from its precursor using afive-step manufacturing process comprising 1. Spinning, 2. Stabilizing,3. Carbonizing, 4. Surface Treatment, and 5. Sizing. These steps are nowdescribed.

In the fiber spinning process, a mixture containing the precursormaterial is spun into individual filaments. The plastic mixture ispumped through small-orifice jets into a chamber where it solidifiesinto filaments. The internal atomic structure of the fiber is formedduring this step. After spinning, the filaments are washed and stretchedto the desired fiber diameter (typically 0.0002 to 0.0003 inch).

During stabilization, the spun polymer filaments are chemically alteredto convert their linear atomic bonding to a more thermally stable ladderbonding. This is accomplished by heating the filaments in air to about390-590° F. (200-300° C.) for 30-120 minutes, which causes them to pickup oxygen molecules from the air and rearrange their atomic bondingpattern.

After stabilization, filaments are carbonized by being heated in afurnace to a temperature of about 1,830-5,500° F. (1,000-3,000° C.) forseveral minutes. The furnace contains an inert gas to prevent the fibersfrom oxidizing. The process drives off the non-carbon atoms in the formof various gases. The remaining carbon atoms form tightly bonded carboncrystals that are aligned along the axis of the filament.

Intermediate modulus and high modulus fibers utilize additionalprocessing after carbonizing, to strip off the outermost layers of thefilament.

After carbonizing, the fiber surface is such that is will not adhere tocomposite resins and matrix materials, so it must undergo a surfacetreatment in which the surface is slightly oxidized.

After the surface treatment, the filaments are coated with a sizingagent that protects it from damage and ensures compatibility with theresin system. The coated fibers are wound onto cylinders called bobbins.The bobbins are loaded into a spinning machine and the filaments arebundled into yarns of various sizes, designated by thousands (K) offibers.

Pricing for a given carbon fiber product may also depend upon the numberof fibers in a yarn. Typical carbon fiber yarn sizes range from 1K to120K, with even larger bundles expected to be available in the future.“Large tow” carbon fibers bundled in the 24K range and up, are lessexpensive than larger diameter fibers comprised of fewer filaments.

Factors other than the strength of particular materials can alsoinfluence the design and construction of cost-effective storage units.For example, the internal structure and expected highest pressure of anenergy storage unit can also impact its design.

The process for manufacturing an energy storage unit can also affect itscost and performance. Certain embodiments may utilize a filament-woundcomposite pressure vessel with a liner, which may or may not beload-sharing. Apart from the liner, other design considerations for thepressure vessel include a thickness of the wall(s) sized to handle theforces expected to result from storage pressures.

In the United States, the American Society of Mechanical Engineers(ASME) Boiler and Pressure Vessel (B&PV) Code is a prevalent standardfor design, fabrication, inspection and operation of installed pressurevessels. The B&PV Code is imposed by many states as an occupationalsafety measure. Section VIII of the B&PV Code applies to metallic andsome (high pressure) composite tanks Section X of the B&PV Code coverssome (low pressure) composite tank designs. Both Section VIII andSection X of the B&PV Code are incorporated by reference in theirentireties herein for all purposes.

ASME B&PV Section X relates to fiber reinforced plastic pressure vesselsmade from carbon or glass fiber. Class I qualifies by testing to 6×design pressure with a burst safety factor of 2.25 for carbon fiber andof 3.5 for glass fiber, and maximum design pressures up to 3000 psi.Class II has mandatory design rules and acceptance tests, with maximumdesign pressures up to ˜100 psi.

Class III of ASME B&PV Section X, including new appendix 8, relates tofully wrapped composite pressure vessels with non-load-sharing liners.These pressure vessels may be intended for hydrogen applications, withmaximum design pressures up to 3600 psi.

New classes of ASME B&PV Section X may relate to fully wrapped compositepressure vessels with load-sharing liners.

FIG. 6 is a simplified view of an example of a tool which may be used tofabricate a composite filament-wound compressed gas storage unitaccording to an embodiment. The tool 650 comprises a mandrel 652 that isrotatable about an axle 654. A roving supply 656 of fiber is fed througha tensioner 658 to a shuttle 660 that is moveable upon a track 662. Theshuttle 660 winds the fiber about the rotating mandrel to create asemi-finished component 664. This semi-finished component is laterfinished through other processes.

A tool employed for filament winding may utilize movement along multipleaxes. For example, a rotational degree of freedom may be represented bythe mandrel, and a linear degree of freedom represented by movement ofthe shuttle.

Other possible axes of motion for filament winding can include but arenot limited to an in/out motion of a winding head and/or a wrist. Stillanother axis of motion may include up/down motion of a winding head.

The embodiment of FIG. 6 shows the fiber as being wet-wound through aresin bath 666, offering the possible benefit of reduced cost. Howeverthis is not required, and in alternative embodiments the resin could bepre-impregnated.

In such pre-impregnation approaches, a fiber bundle (such as a tow,strand, roving, tape, fabric, or yarn) is exposed to the resin prior tothe winding process. At this time, the fiber bundle becomes impregnatedwithin the resin. An initial heat curing step of the impregnated fiberbundle may then be performed, prior to storing the impregnated fiber(typically at lower than room temperature).

In some embodiments the pre-impregnated fiber may be stored by windingaround a spool. An intervening material may or may not be employedbetween the pre-impregnated fiber layers, in order to prevent theiradhesion to one another during storage.

The pre-impregnated fiber bundle may then be wound around the mandrel. Asecond heat cure of the wound filament/mandrel combination, serves tosecure the wound filament to the mandrel.

Such a pre-impregnated fiber approach may offer certain benefits. Onepossible benefit is uniformity of resin impregnation of the fiberbundle, as compared with wet approaches. Another possible benefit isreduced exposure by workers to wet chemicals and fumes, during thefilament winding process itself.

A variety of materials may be used as a resin in a filament wound tankaccording to various embodiments. Three types of thermosetting polymersinclude polyesters, vinyl esters, and epoxies. These are the three mostcommon generic chemical families of resins used for filament winding.

Unsaturated polyesters are the most widely used and inexpensive windingresins on the market, with costs as low as $1 per lb. They are widelyused for winding fiberglass pressure vessels and piping. Mechanicalproperties and chemical resistance is the lowest of the three types ofresins.

Vinyl esters offer improved mechanical properties and chemicalresistance. However, recent regulatory requirements in the United Stateshave reduced the allowable amount of styrene emissions, and impose anincreasingly stringent, costly, and time-consuming process forpermitting and demonstrating compliance.

Despite the fact their relative expense, epoxies offer favorablemechanical properties. One epoxy resin system uses Epon Resin 826 withEpikure 9551 curing agent (both from Momentive Specialty Chemicals),currently priced at $2.46 per lb. Perhaps more importantly, epoxy resinsystems are a more environmentally responsible choice since they havemuch a lower emissions factor than polyesters, do not contain styrene,and are not affected by the newest round of regulations.

Certain resin types may offer resistance to corrosion, a property whichmay be beneficial in environments containing water. Examples of suchresins include but are not limited to phenolic, chlorendic, andbisphenol-A fumarate.

Thermoplastic materials may be used in resins. Thermoplastic materialsmay be cured by the application of heat. Examples of possiblethermoplastic resins include but are not limited to:

polyamide-imide;

polyetheretherketone;

polyetherketoneketone;

polyarylsulfone;

polyether imide;

polyethersulfone; and

liquid crystal polymer.

In certain embodiments, the compressed gas storage unit may include aliner whose role is to form a gas-tight seal, with the surrounding woundfiber providing the necessary physical strength. The liner may or maynot be load sharing. An example of a load sharing liner is one made froma metal such as aluminum, having a threshold wall thickness. An exampleof a non-load sharing liner may be one made from plastic and notexhibiting the requisite strength.

Embodiments of composite compressed gas storage units formed by filamentwinding, may be expected to contain gas a high pressures. Examples ofinternal gas pressures that the filament wound compressed gas storageunits may be expected to withstand, including but are not limited to 100psig, 200 psig, 250 psig, 500 psig, 1000 psig, 1500 psig, 2000 psig,2500 psig, 3000 psig, 3500 psig, 4000 psig, 4500 psig, 5000 psig, 5500psig, and 6000 psig, 7000 psig, and 10,000 psig. Examples of internalvolumes which may be defined within compressed gas storage units formedby blow molding, include but are not limited to 0.002 m³, 0.005 m³, 0.01m³, 0.025 m³, 0.05 m³, 0.1 m³, 0.5 m³, 1 m³, 2 m³, 2.5 m³, 3 m³, 4 m³, 5m³, 10 m³.

According to certain embodiments, a plastic liner may be used. Incertain embodiments such a plastic liner may comprise a thermosetmaterial.

In some embodiments, a plastic liner may comprise a thermoplasticmaterial. Such a thermoplastic material may allow welding or a heatsealing process to join two separate workpieces at their interface.

In some embodiments the liner may be formed from a plastic materialhaving a desirable combination of one or more characteristics. Anexample of a desirable characteristic for a liner material is lowflammability at the high gas pressures expected within the unit. Forexample, compressed air at 3000 psi (207 bar) and 4350 psi (300 bar) isclassed as an enriched oxygen-rich environment, because the partialpressure of the oxygen exceeds a threshold value of 27.5 kPa. Thus thematerial for the liner may be chosen so as not be flammable under theseconditions.

Examples of other potentially desirable characteristics may include butare not limited to thermal properties allowing fabrication of the liner,and weldability. One specific possible candidate plastic materialexhibiting a desirable combination of properties is high densitypolyethylene (HDPE).

In certain embodiments, the liner may form part or all of the rotatablemandrel about which the fiber is wound. In cases involving compressedgas storage units of significant length, longitudinal fibers can bemolded into the liner or applied to the outer diameter of the liner inorder to prevent sagging of the liner/mandrel during the windingprocess, and/or the liner could be reinforced by the insertion of acollapsible stiffener. As shown in FIG. 7, this collapsible stiffener702 could be collapsed to be inserted through the opening 704 in theliner 706, and then expanded to contact the sides of the liner andprovide physical support. Following the winding and curing process, thestiffener could be collapsed and retracted from inside the liner, whosecontinued stiffness is now ensured by the presence of the woundfilament.

The composition and structure of internal components such as liners,resins, and fibers, can affect the performance of a gas storage unit.The expense of such a storage unit can be reduced by allocatinghigher-cost materials to key roles, while allocating lower-costmaterials to other roles. Thus according to various embodiments, acompressed gas storage unit may exhibit a heterogeneous structurecomprising more than just the liner and a single type of fiber winding.

Appropriate fibers may be evaluated and implemented to reduce cost.Examples of such fibers include large tow, commercial grade carbon asdescribed above. Such fiber offers high strength (4 to 5× higher thanSAE 4340 steel), and light weight (35% lighter than aluminum, 80%lighter than steel).

Wound filaments may also comprise polymer material. In the fiber form,UHMWPE has a maximum tensile strength of about 435,000 psi. By contrast,in the solid form the tensile strength of UHMWPE is about two orders ofmagnitude lower, or 4000 psi. Calculation of the wall thickness ast=Pr/S, (P=internal pressure, r=radius, S=strength) indicates therelationship as being inversely linear. Accordingly, a tank wall of UHMWPE can be 100 times thinner if the fiber form is used instead of solidplastic.

Other candidates for fiber materials include basalt, fiberglass, andKevlar. Such fibers may offer high strength (3× higher than SAE 4340steel) and light weight (similar to aluminum, 65% less than steel) atrelatively low cost. However, the stress-strain response of such fibersis highly nonlinear, and the ASME codes may require higher safetyfactors (3.5) for these fibers than for carbon (2.4), potentiallyincreasing cost.

Still another candidate for a fiber material is steel wire. The use ofsteel wire as a wound filament in a pressure vessel is described indetail in U.S. patent application Ser. No. 13/050,442, filed Mar. 17,2011 and incorporated by reference in its entirety herein for allpurposes. While offering the potential benefit of low cost fiber,embodiments utilizing a wound steel wire may not covered by existingcodes, potentially affecting development costs.

The following equation provides a Figure of Merit (FOM) as a measure toassess relative costs of filament-wound compressed gas storage unitsmade from various materials, with a lower FOM representing a lower costsolution:

${{FOM} = {\left( \frac{SF}{\overset{\_}{\sigma_{f}}} \right)\left\lbrack {\$_{f} + {{\$_{r}\left( \frac{1 - v_{f}}{v_{f}} \right)}\left( \frac{\rho_{r}}{\rho_{f}} \right)}} \right\rbrack}},$

where:

SF=Safety Factor

σ=Specific Strength of fiber$_(f)=Unit Price of Fiber$_(r)=Unit Price of Resinν_(f)=Fiber Volume Fractionρ_(r)=Resin Densityρ_(f)=Fiber Density

FIG. 8 shows the results of applying the FOM to a number of differentfiber types having the characteristics shown in FIG. 9, to a filamentwound structure having the same dimensions. The results in FIG. 9 arebased upon an assumption of a fiber volume fraction of 0.6 and a unitprice of resin of 2.55 ($/lb). FIG. 26 shows a list of actual costs (in$/kWhr) of various materials, with a steel tank included for reference.

While the above figures have shown the fabrication of compressed gasstorage tanks by blow molding or fiber winding, these processes are notexclusive. A compressed gas storage unit according to some embodimentsmay feature a gas tight internal plastic liner formed by blow molding,reinforced to resist high internal pressures by filament winding.

Some embodiments may allow a reduction in the required safety factor,where a strength of the blow molded liner allows it to share a part ofthe load. In certain embodiments the blow molded liner may be formedutilizing a filament wound structure as a mold. According to someembodiments, a stiffness/strength of a blow-molded liner may facilitateits use as a mandrel in a filament winding process.

According to still other embodiments, a liner may be molded utilizingother techniques inside a filament-wound or other type of pressurevessel that functions as the primary load-bearing structure. For examplesome embodiments could employ a rotational molding approach rather thanblow-molding. The liner could serve as sealing element to preventleakage.

Embodiments of gas storage units could be provided with a “leak beforeburst” (LBB) safety feature in order to avoid explosion in the eventthat a tank were to become overpressurized. In one such embodiment, theLBB feature could be that a molded plastic liner includes a safetyfeature to ensure that the liner will leak at a pressure lower than theburst pressure of a composite overwrap. According to alternativeembodiments, an LBB feature could be imparted by assembling a moldedplastic liner by joining two or more pieces in such a way that the jointbetween them will leak at a pressure lower than a burst pressure of acomposite overwrap.

Whether fabricated by blow molding, rotational molding, filamentwinding, and/or other techniques, embodiments of compressed gas storageunits according to the present invention may be bundled to form largermodules exhibiting desirable characteristics. One example of such acharacteristic is size, where the individual compressed gas storageunits may be assembled into a module that is sized to fit on a flatbedtrailer of a standard tractor-trailer rig. Other examples of possibleform factors include the size of a standard enclosed trailer of atractor-trailer, the size of a standard railroad box car, the size of astandard railroad flatbed car, or the size of a standard shippingcontainer.

Still another possible characteristic of a compressed gas storage moduleis capacity. In particular, in some embodiments a module may beconfigured to contain a sufficient amount of compressed gas to deliver astandard increment of power (e.g. 1 MWh of power). The highest pressureof the gas contained within the storage unit could be 200 bar, 300 bar,or even higher depending on the embodiment.

For example, at 300 bar, about 30 liters of compressed gas storage maybe needed per kWh. At 200 bar, 50 l/kWhr may be needed. Thus acompressed gas storage module having a capacity of 1 MWh of storage mayutilize a minimum volume of 40 m³.

FIG. 10 shows an embodiment of a compressed gas storage module. Module1000 contains six (6) individual storage units 1002 having a cylindricalshape. Each storage unit may have an internal volume of about 2 m³ andan air storage pressure of about 200 bar (˜3000 psi). The dimensions ofthe storage units are approximately 45″ diameter, and less than 48′long. The individual storage units may be in selective fluidcommunication through external piping 1004.

According to certain embodiments, the end fixtures ofcylindrically-shaped storage units could include features such asflanges and/or threads to facilitate connection with another storageunit, thereby allowing storage capacity to be expanded or reduced in amodular manner. In certain embodiments the connections betweensuccessive storage units could have a particular shape, to allowarrangement of the tanks in compact serpentine or folded configurations.

For example, FIG. 29A shows a side elevational view of a configurationutilizing a vertical folded configuration for pressure vessels. FIG. 29Bshows a plan view of a configuration utilizing a serpentine horizontalfolded configuration. Such configurations may employ elbow-typeconduits. As used herein the term elbow is not limited to a shapeexhibiting any particular angle (such as 90°), but instead encompasses aconduit whose main axis experiences a change in direction along itslength.

Elbow-type conduits themselves may also be fabricated utilizing bulkmaterial (such as steel or another metal), or may be fabricatedutilizing composite materials such as filament-wound designs. Examplesof elbow-type conduits utilizing filament winding principles, aredescribed by Li and Liang in “Computer aided filament winding forelbows”, J. Software, Vol. 13(4), pp. 518-25 (2002), which isincorporated by reference in its entirety herein for all purposes.

Whether fabricated by blow molding, filament winding, and/or othertechniques, embodiments of compressed gas storage units may beconfigured to include elements to perform heat exchange with thecompressed gas. FIG. 11 shows one embodiment, wherein heat exchangeelement 1100 comprising fins 1102, extends inside the storage unit 1104.

A heat exchange fluid is flowed through the heat exchange element 1100by circulator 1106. In certain embodiments, the heat exchange fluid maybe a liquid, such as water. In some embodiments the heat exchange fluidmay be a gas, such as helium. In particular embodiments, the heatexchange fluid may undergo a phase change between the liquid and gas.

The circulated heat exchange fluid is in thermal communication with anexternal heat source/heat sink 1108 through a heat exchanger 1110 inorder to receive/dissipate thermal energy. For example, where compressedgas is being flowed into the unit for storage, the heat exchanger may beplaced into thermal communication with a heat sink. Where compressed gasis being flowed out of the unit, the heat exchanger may be placed intothermal communication with a heat source.

FIGS. 11AA-AB show perspective and cross-sectional views of anotherembodiment of a compressed gas storage unit including an internal heatexchange element. This particular embodiment comprises a plurality ofcircular fins oriented orthogonal to the central tube through which theexchange fluid flows. One or more portions of an internal heat exchangeelement may be constructed from corrosion-resistant material, of whichstainless steel represents only one particular example. According tosome embodiments, an internal heat exchange structure (e.g. a finnedtube) may also function to provide longitudinal support and preventsagging of a mandrel (which can be a liner) during winding.

A heat source may be naturally-occurring (e.g. solar radiation,geothermal) or artificial (e.g. the result of an industrial process,power generation, energy storage, or other human activity). A heat sinkmay be naturally-occurring (e.g. an existing body of water such as alake or the ocean, or an environmental temperature) or artificial (e.g.a cooling tower).

Certain embodiments may comprise one or more heat pipe structures to aidin the efficient transmittal of thermal energy to/from the compressedgas present within the storage unit. Such heat pipe structures may relyupon a phase change of a heat exchange fluid circulated for selectiveexposure to a heat source and/or heat sink. The heat exchange fluid maybe actively circulated (e.g. by a pump and/or fan), or may be passivelycirculated (e.g. by capillary and/or gravitational action) in whole orin part.

In some embodiments, heat exchange may involve the introduction of aliquid directly into the compressed gas storage unit for heat exchangewith the compressed gas. One such embodiment is shown in FIG. 12, wherecirculating liquid 1200 flowed through central pipe 1202 is sprayedthrough openings 1204 into the interior 1206 of the storage unit 1208for heat exchange, and then is drawn for circulation by pump 1210through drain 1212.

According to certain embodiments, liquid water may be sprayed as a heatexchange medium. In some embodiments, liquid nitrogen may be sprayed asa heat exchange medium. Such an embodiment may offer benefits in thatliquid nitrogen is very cold and could also serve to inert compressedair by reducing its oxygen content, for improved safety.

The compressed gas storage unit may be specifically designed toaccommodate a heat exchanger structure. For example, as shown in FIG. 13end pieces 1300 located at poles 1302 of a storage unit 1304 orientedalong axis A, may be of a larger size to accommodate insertion of theheat exchanger. Such a configuration may offer the additional benefit offacilitating construction by reducing the extent E of curved surfaceslocated at the poles that may be difficult to effectively coverutilizing conventional filament winding geometries.

Moreover, in some embodiments the heat exchanger may change shape oncepositioned within the interior of the storage unit. For example, asshown in FIG. 14, projections 1400 (for example serving as fins) of theheat exchange element 1402 may unfold once positioned inside thecompressed gas storage unit 1404, under the influence of mechanicalaction. Alternative embodiments could rely upon other forces toaccomplish such a shape change, for example pressure of a flowed heatexchange fluid.

While it may be desirable for the heat exchanger to also be collapsibleallowing removal from the compressed gas storage unit for periodicinspection, maintenance, and/or replacement, this is not required in allembodiments. Moreover, in certain embodiments a shape changing structurecould serve more than one purpose, for example providing support duringfabrication (similar to FIG. 7), and then remaining in the completedstructure so as to perform a heat exchange function.

According to certain embodiments, a liner of a compressed gas storageunit may perform a heat exchange function. For example the liner maycomprise a thermally conducting material (e.g. a metal such as aluminumor thermally conductive polymer), which depending upon its thickness mayor may not also serve a load-sharing function.

Thermal energy could be communicated passively or actively to and fromcompressed gas within the storage unit via the liner, for exampleutilizing a heat exchange medium comprising a liquid, solid, and/or gas.The thermal energy could communicate with the liner through walls of thestorage unit, through a port, and/or through members extending throughthe walls.

FIG. 15 shows a particular embodiment wherein thermal energy may becommunicated to liner 1500 of compressed gas storage unit 1502 viamembers 1504 extending through walls 1506. Members 1504 may be solid ormay exhibit a more complex structure (e.g. perforated, hollow, sealedheat pipe), and may be in selective communication with a heat sinkand/or heat source 1508 utilizing heat exchange fluid 1510 (e.g. gas,liquid, phase changing material). In this particular embodiment the heatexchange fluid may be actively flowed by circulator 1512 (e.g. fan orpump), but in other embodiments may flow passively wholly or in part(e.g. by mechanisms such as convection, conduction, and/or radiation).

According to certain embodiments, the liner itself may comprise multipleelements. For example, in the particular embodiment of FIG. 16, tubing1600 (which may be solid or hollow) comprising aluminum (or anotherthermally conductive material) could comprise a middle component of asandwich 1602 comprising an inner liner 1604, the tubing, and an outerliner 1606.

According to some embodiments, a liner could be fabricated with coolantchannels built in. For example, in particular embodiments a liner couldbe fabricated from a plurality of blow molded shells having anintervening heat exchange medium present therebetween.

FIG. 17 shows an embodiment of a liner 1700 cast with channels 1702 inthe outside surface 1704, and then covered with a skin 1706 that sealsthe channels to retain a heat exchange medium 1708. Such channels couldassume many configurations, including circumferential, longitudinal,spiral, interconnected, or any combination thereof.

To aid in the heat exchange function, certain embodiments could includeadditional elements configured to promote thermal interaction between aliner and compressed gas within the storage unit. As shown in FIG. 18,such elements could operate in a passive or active manner to enhancethermal interaction with a liner 1800 enclosed within a wall 1801 of acompressed gas storage unit. Examples of such structures may include butare not limited to airfoils 1802, baffles 1804, turbines 1806, fans1808, or heat pipes 1810.

Example

An embodiment of a design for a compressed gas storage unit employs afully overwrapped pressure vessel made from standard modulus, large towcarbon fiber in an epoxy matrix. The storage unit includes a polymerliner with an embedded metal fitting at each pole. An axisymmetric sliceof the full-scale tank is shown in FIG. 19, and a detailed view of theport and dome region is shown in FIG. 20.

The length of the cylindrical portion of the tank is 40 feet, measuredbetween the dome tangency points. An isotensoid geodesic dome contourwas used. The isotensoid contour may be preferred over hemispherical orsemi-elliptical shapes for filament wound tanks, because it may provideuniform tensile stresses in the helical fiber at all points on the fiberpath of the dome. The isotensoid contour may also be more spatiallyefficient than a hemispherical dome. The height of the each dome isapproximately 13 inches.

A liner thickness of about 0.25 inches can be optimized. An embeddedmetal fitting with a polar opening was used in the preliminary design.The diameter of the fitting is large enough to prevent the fitting fromshearing thru the composite, while the port openings and flanges areconsistent with a standard flanged pressure fitting. The fitting contourmay be optimized. Port openings were used at both ends of the tank.

The friction angle parameter for a wet-wound composite structure definesan upper bound for the orientation of helical fibers to ensure that theydo not slip off the dome during the winding process. If the wind angleexceeds the friction angle, friction is required to ensure that thefibers are stable. While the friction angle may be exceeded intow-pregnated vessels, this parameter can cause problems with a slipperyresin-coated fiber.

The friction angle on an isotensoid dome can be determined geometricallyas the arcsine of the ratio of port and cylinder openings. The effectiveport radius on the baseline liner may be 2.99 inches, while the innerdiameter of the cylinder section was 19 inches. Accordingly, thecalculated helical angle for the baseline tank was 9.06 degrees. Thefollowing table contains a summary of the composite layup and plystacking sequence for a baseline tank.

Orientation Number of plies Thickness Angle Hoop 2 0.0532 89.25 Helical1 pair (+/−) 0.0528 +/−9.06 Hoop 2 0.0532 89.25 Helical 1 pair (+/−)0.0528 +/−9.06 Hoop 2 0.0532 89.25 Helical 1 pair (+/−) 0.0528 +/−9.06Hoop 2 0.0532 89.25 Helical 1 pair (+/−) 0.0528 +/−9.06 Hoop 2 0.053289.25 Helical 1 pair (+/−) 0.0528 +/−9.06

Preliminary analysis of the baseline tank design was completed to verifythe structural integrity during operating conditions. Finite elementanalysis of the 3000 psi pressure load was completed using MSC NASTRAN,and results showed that positive margins of safety will be maintainedfor a composite tank with a 1.01 inch thick wall.

Half of the tank was modeled, and symmetry conditions were applied atits midpoint. The model used 12,736 nodes and 15,552 shell elements. 43different layups were represented as 2D laminate elements using PCOMPcards in NASTRAN. Solid elements were used to model the metal fittings.FIG. 21 shows the finite element model of half the tank, and FIG. 22provides a closer view of the model in the area of the dome and metalfitting.

Data for fiber and resin was used to calculate composite properties forthe hoop and helical plies using micromechanics and assuming a squarepacking array. A fiber volume fraction of 0.61 was used for the hoops,and 0.55 was used for the helicals. The following table 4 summarizesdatasheet and assumed properties for Zoltek Panex 35 fiber.

Fiber Property Value Unit Notes Density 0.0654 Lb/in³ Zoltek data sheetModulus E₁ 35,100 Ksi Zoltek data sheet Tensile Strength 600 Ksi Zoltekdata sheet Poisson ratio ν₁₂ 0.2 — From T300 3K Fiber data TensileModulus E₂, E₃ 2340 Ksi From ratio of E₁/E₂ for T300 Shear Modulus G₁₂,G₁₃ 4250 Ksi From ratio of E₁/G₁₂ for T300 Shear Modulus G₂₃ 1060 KsiFrom ratio of G₁₂/G₂₃ for T300

The following table contains data for 3M Resin 4831.

Resin Property Value Unit Notes Density 0.051 Lb/in³ 3M data sheetTensile Modulus 722 Ksi 3M data sheet Shear Modulus 268 Ksi 3M datasheet Tensile Strength 12.8 Ksi 3M data sheet Poisson Ratio ν₁₂ 0.347 —3M data sheet Compressive Strength 19.2 Ksi Estimated Shear Strength 9.5Ksi Estimated

The calculated input properties for the hoop plies are provided in thefollowing table.

Calculated Property Value Unit Notes Density 0.0598 Lb/in³ Calculatedusing Chamis Tensile Modulus E₁ 21,700 Ksi micromechanics model inTensile Modulus E₂, E₃ 1540 Ksi ABAQUS with square Poisson Ratio ν₁₂,ν₁₃ 0.252 packing array and 61% Poisson Ratio ν₂₃ 0.451 fiber volumefraction. Shear Modulus G₁, G₂ 19.2 Ksi Shear Modulus G₃ 9.5 Ksi TensileStrength-1 dir 371 Ksi Compressive Strength-1 dir 295 Ksi From ratio ofT300 at 60% FVF Tensile Strength-2 dir 11.3 Ksi Compressive Strength-2dir 37.6 Ksi Shear Strength 6.6 Ksi Interlaminar Shear Strength 11.3 Ksi

Calculated properties for the helical plies are in the following table.

Calculated Property Value Unit Notes Density 0.0589 Lb/in³ Calculatedusing Chamis Tensile Modulus E₁ 19600 Ksi micromechanics model inTensile Modulus E₂, E₃ 1440 Ksi ABAQUS with square Poisson Ratio ν₁₂,ν₁₃ 0.455 packing array and 55% Poisson Ratio ν₂₃ 0.387 fiber volumefraction. Shear Modulus G₁, G₂ 794 Ksi Shear Modulus G₃ 435 Ksi TensileStrength-1 dir 335 Ksi Compressive Strength-1 dir 267 Ksi From ratio ofT300 at 55% FVF Tensile Strength-2 dir 11.1 Ksi Compressive Strength-2dir 39 Ksi Shear Strength 7.5 Ksi Interlaminar Shear Strength 11.1 Ksi

Loads and boundary conditions were determined as follows. An internalpressure load of 3000 psi was the only external load applied to themodel. Symmetry boundary conditions were imposed along the mid-plane ofthe cylinder, since only half of the tank was modeled.

Analysis showed that positive margins of safety were maintainedthroughout the tanks, although stress concentrations were observed intwo transition areas. Ply-by-ply failure indices were computed using adesign safety factor of 2.25. A failure index of 1.0 represents a marginof safety of 0.0. If the index is greater than 1.0, the margin isnegative. Failure indices less than 1.0 represent positive margins ofsafety.

In the cylinder portion of the tank, the failure index for the hoopfibers was 0.99, occurring in the inner-most hoop fiber. The failureindex for the helicals was 0.62. Failure indices that exceeded 1.0occurred in two transition areas, denoting stress concentrations inthose locations. The first of these hot spots occurred in the helicalfibers of the dome at the outermost edge of the metal fitting. Thesecond area of concern was the tangency area at the intersection of thedome and the cylinder. These locations can be addressed by further minordesign modifications.

At the edge of the metal fitting, changes to the shape of the fittingshould be able to ensure a smoother transition to minimize (oreliminate) stress peaks in that area. If not, local reinforcement suchas doilies may be required. In the tangency areas, the locations of plydrops for the hoops can be adjusted to smooth out the stress peaks.

FIG. 23 shows a plot of failure indices in the full model of the tankdue to the 3000 psi pressure load, while FIG. 24 shows the same plotwith peaks omitted in order to provide a more representative contourplot for visualization.

FIG. 25 shows a magnified plot of deformation in the dome underpressure. Stresses at the edge of the metal fitting can be reduced bymodification of the fitting design to reduce the inflection points thatoccur when the dome is pressurized.

22. A compressed gas storage unit comprising:

a molded plastic liner defining an internal chamber; and

a wound filament in contact with the molded plastic liner.

23. A compressed gas storage unit as in clause 22 wherein the moldedplastic liner comprises a blow molded shell.

24. A compressed gas storage unit as in any of the previous clauseswherein the molded plastic liner is molded within the wound filament.

25. A compressed gas storage unit as in any of the previous clauseswherein the molded plastic liner is rotary molded.

26. An apparatus comprising:

a compressed gas storage unit defining an internal chamber; and

a heat exchanger disposed in the internal chamber to communicate heat toand from stored compressed gas.

27. An apparatus as in clause 26 wherein the heat exchanger comprises acirculating heat exchange fluid.

28. An apparatus as in any of the previous clauses wherein the heatexchanger comprises a heat pipe.

29. An apparatus as in any of the previous clauses wherein thecirculating heat exchange fluid is thermal communication with a heatsource and/or heat sink through an external heat exchanger.

30. An apparatus as in any of the previous clauses wherein thecirculating heat exchange fluid is actively circulated.

31. An apparatus as in any of the previous clauses wherein thecirculating heat exchange fluid comprises a liquid.

32. An apparatus as in any of the previous clauses wherein the heatexchanger is configured to introduce the liquid into the chamber, andthe compressed gas storage unit further comprises a drain.

33. A compressed gas storage module comprising a plurality of compressedgas storage units in selective fluid communication, each compressed gasstorage unit having a dimension to allow the compressed gas storagemodule to conform to a standard form factor.

34. A compressed gas storage module as in clause 33 wherein the standardform factor allows the compressed gas storage module to be transportedby road.

35. A compressed gas storage module as in any of the previous clauseswherein the plurality of compressed gas storage units is fabricated byblow molding.

36. A compressed gas storage module as in any of the previous clauseswherein the plurality of compressed gas storage units is fabricated byfilament winding.

37. A compressed gas storage module as in any of the previous clauseswherein the plurality of compressed gas storage units are configured tobe in selective fluid communication with an energy storage device, andare sized to provide a standard capacity of output power.

Compressed gas storage units according to various embodiments may findparticular use in the storage of large volumes of compressed gas inconjunction with energy storage, for example as described in the U.S.Patent Publication No. 2011/0115223 (“the '223 Publication”). Thisdocument shows a number of embodiments of compressed gas energy storagesystems, including systems utilizing multiple successive expansionand/or compression stages.

FIG. 27 shows a simplified view of one embodiment of such a compressedgas energy system. In particular, the system 2700 includes acompressor/expander 2702 comprising a cylinder 2704 having piston 2706moveably disposed therein. The head 2706 a of the piston is incommunication with a motor/generator 2708 through a piston rod 2706 band a linkage 2710 (here a crankshaft).

In a compression mode of operation, the piston may be driven by themotor/generator 2705 acting as a motor to compress gas within thecylinder. The compressed gas may be flowed to a gas storage tank 2770,or may be flowed to a successive higher-pressure stage for additionalcompression.

In an expansion mode of operation, the piston may be moved by expandinggas within the cylinder to drive the motor/generator acting as agenerator. The expanded gas may be flowed out of the system, or flowedto a successive lower-pressure stage for additional expansion.

The cylinder is in selective fluid communication with a high pressureside or a low pressure side through valving 2712. In this particularembodiment, the valving is depicted as a single multi-way valve.However, the present invention is not limited to such a configuration,and alternatives are possible.

For example, in lieu of a single, multi-way valve, some embodiments ofthe present invention may include the arrangement of multiple one-way,two-way, or three-way valves in series. Examples of valve types whichcould be suitable for use in accordance with embodiments of the presentinvention include, but are not limited to, spool valves, gate valves,cylindrical valves, needle valves, pilot valves, rotary valves, poppetvalves (including cam operated poppet valves), hydraulically actuatedvalves, pneumatically actuated valves, and electrically actuated valves(including voice-coil actuated valves).

Certain embodiments may employ gas flow valves as have been employed insteam engine design. Examples of such valves include slide valves (suchas D valves), Corliss valves, and others as are described by JoshuaRose, M. E, in Modern Steam Engines, Henry Carey Baird & Co.,Philadelphia, Pa. (1887), reprinted by Astragal Press (2003), which isincorporated by reference in its entirety herein for all purposes.

When operating in the compression mode, gas from the low pressure sideis first flowed into the cylinder, where it is compressed by action ofthe piston. The compressed gas is then flowed out of the cylinder to thehigh pressure side.

When operating in the expansion mode, gas from the high pressure side isflowed into the cylinder, where its expansion drives the piston. Theexpanded gas is subsequently exhausted from the cylinder to the lowpressure side.

Embodiments of the present invention utilize heat exchange betweenliquid and gas that is undergoing compression or expansion, in order toachieve certain thermodynamic efficiencies. Accordingly, the systemfurther includes a liquid flow network 2720 that includes pump 2734 andvalves 2736 and 2742.

The liquid flow network is configured to inject liquid into the cylinderto perform heat exchange with expanding or compressing gas. In thisembodiment, the liquid is introduced through nozzles 2722. In otherembodiments, a bubbler may be used, with the gas introduced as bubblesthrough the liquid.

The liquid that has been injected into the cylinder to exchange heatwith compressed gas or expanding gas, is later recovered by gas-liquidseparators 2724 and 2726 located on the low- and high-pressure sidesrespectively. Examples of gas-liquid separator designs include verticaltype, horizontal type, and spherical type. Examples of types of suchgas-liquid separators include, but are not limited to, cycloneseparators, centrifugal separators, gravity separators, and demisterseparators (utilizing a mesh type coalescer, a vane pack, or anotherstructure).

Liquid that has been separated may be stored in a liquid collectorsection (2724 a and 2726 a respectively). A liquid collector section ofa separator may include elements such as inlet diverters includingdiverter baffles, tangential baffles, centrifugal, elbows, wavebreakers, vortex breakers, defoaming plates, stilling wells, and mistextractors.

The collected separated liquid is then thermally conditioned forre-injection. This thermal conditioning may take place utilizing athermal network. Examples of components of such a thermal networkinclude but are not limited to liquid flow conduits, gas flow conduits,heat pipes, insulated vessels, heat exchangers (including counterflowheat exchangers), loop heat pipes, thermosiphons, heat sources, and heatsinks.

For example, in an operational mode involving gas compression, theheated liquid collected from gas-liquid separator 2726 is flowed throughheat exchanger 2728 that is in thermal communication with heat sink2732. The heat sink may take one of many forms, including an artificialheat sink in the form of a cooling tower, fan, chiller, or HVAC system,or natural heat sinks in the form of the environment (particularly athigh latitudes or altitudes) or depth temperature gradients extant in anatural body of water.

In an operational mode involving gas expansion, the cooled liquidcollected from gas-liquid separator 2724 is flowed through heatexchanger 2752 that is in thermal communication with heat source 2730.Again, the heat source may be artificial, in the form of heat generatedby industrial processes (including combustion) or other man-madeactivity (for example as generated by server farms). Alternatively, theheat source may be natural, for example geothermal or solar in nature(including as harnessed by thermal solar systems).

Flows of liquids and/or gases through the system may occur utilizingfluidic and/or pneumatic networks. Examples of elements of fluidicnetworks include but are not limited to tanks or reservoirs, liquid flowconduits, gas flow conduits, pumps, vents, liquid flow valves, gas flowvalves, switches, liquid sprayers, gas spargers, mixers, accumulators,and separators (including gas-liquid separators and liquid-liquidseparators), and condensers. Examples of elements of pneumatic networksinclude but are not limited to pistons, accumulators, gas chambersliquid chambers, gas conduits, liquid conduits, hydraulic motors,hydraulic transformers, and pneumatic motors.

As shown in FIG. 27, the various components of the system are inelectronic communication with a central processor 2750 that is incommunication with non-transitory computer-readable storage medium 2754,for example relying upon optical, magnetic, or semiconductingprinciples. The processor is configured to coordinate operation of thesystem elements based upon instructions stored as code within medium2754.

The system also includes a plurality of sensors 2760 configured todetect various properties within the system, including but not limitedto pressure, temperature, volume, humidity, and valve state. Coordinatedoperation of the system elements by the processor may be based at leastin part upon data gathered from these sensors.

The particular system shown in FIG. 27 represents only one particularembodiment, and alternatives having other features are possible. Forexample, while FIG. 27 shows an embodiment with compression andexpansion occurring in the same cylinder, with the moveable element incommunication with a motor/generator, this is not required.

FIG. 28 shows an alternative embodiment utilizing two cylinders, whichin certain modes of operation may be separately dedicated forcompression and expansion. Embodiments employing such separate cylindersfor expansion and compression may, or may not, utilize a common linkage(here a mechanical linkage in the form of a rotating crankshaft) with amotor, generator, or motor/generator.

For example, FIG. 28A is a table showing four different basicconfigurations of the apparatus of FIG. 28. The table of FIG. 28Afurther indicates the interaction between system elements and variousthermal nodes 28625, 28528, 28530, 28532, 28534, 28536, and 28540, inthe different configurations. Such thermal nodes can comprise one ormore external heat sources, or one or more external heat sinks, asindicated more fully in that table. Examples of such possible suchexternal heat sources include but are not limited to, thermal solarconfigurations, geothermal phenomena, and proximate heat-emittingindustrial processes. Examples of such possible such external heat sinksinclude but are not limited to, the environment (particularly at highaltitudes and/or latitudes), and geothermal phenomena (such as snow orwater depth thermal gradients).

FIGS. 28BA-28BD are simplified views showing the various basicoperational modes listed in FIG. 28A. The four different basic modes ofoperation shown in FIG. 28A may be intermittently switched, and/orcombined to achieve desired results. FIGS. 28BE-BF show operationalmodes comprising combinations of the basic operational modes.

One possible benefit offered by the embodiment of FIG. 28 is the abilityto provide cooling or heating on demand. Specifically, the change intemperature experienced by an expanding or compressed gas, or aninjected liquid exchanging heat with such an expanding or compressedgas, can be used for temperature control purposes. For example, gas orliquid that is cooled by expansion, could be flowed to a building HVACsystem. Conversely, the increase in temperature experienced by acompressed gas, or a liquid exchanging heat with a compressed gas, canbe used for heating.

By providing separate, dedicated cylinders for gas compression orexpansion, embodiments according to FIG. 28 may provide such temperaturecontrol on-demand, without reliance upon a previously stored supply ofcompressed gas. In particular, the embodiment of FIG. 9 allows coolingbased upon immediate expansion of gas compressed by the dedicatedcompressor.

While FIGS. 27-28 show embodiments involving the movement of a solid,single-acting piston, this is not required. Alternative embodimentscould utilize other forms of moveable elements. Examples of suchmoveable elements include but are not limited to double-acting solidpistons, liquid pistons, flexible diaphragms, screws, turbines,quasi-turbines, multi-lobe blowers, gerotors, vane compressors, andcentrifugal/axial compressors. Where a solid piston is used, a pistonrod and/or crosshead may also be employed.

Moreover, embodiments may communicate with a motor, generator, ormotor/generator, through other than mechanical linkages. Examples ofalternative linkages which may be used include but are not limited to,hydraulic/pneumatic linkages, magnetic linkages, electric linkages, andelectro-magnetic linkages.

While the particular embodiments of FIGS. 27-28 show a piston incommunication with a motor generator through a mechanical linkage in theform of a crankshaft, this is not required. Alternative embodimentscould utilize other forms of mechanical linkages, including but notlimited to gears such as multi-node gearing systems (including planetarygear systems). Examples of mechanical linkages which may be used includeshafts such as crankshafts, gears, chains, belts, driver-followerlinkages, pivot linkages, Peaucellier-Lipkin linkages, Sarrus linkages,Scott Russel linkages, Chebyshev linkages, Hoekins linkages, swashplateor wobble plate linkages, bent axis linkages, Watts linkages, trackfollower linkages, and cam linkages. Cam linkages may employ cams ofdifferent shapes, including but not limited to sinusoidal and othershapes. Various types of mechanical linkages are described in Jones in“Ingenious Mechanisms for Designers and Inventors, Vols. I and II”, TheIndustrial Press (New York 1935), which is hereby incorporated byreference in its entirety herein for all purposes.

In certain embodiments of the present invention, it may be important tocontrol the amount of liquid introduced into the chamber to effect heatexchange. The ideal amount may depends on a number of factors, includingthe heat capacities of the gas and of the liquid, and the desired changein temperature during compression or expansion.

The amount of liquid to be introduced may also depend on the size ofdroplets formed by the spray nozzle. One measure of the amount of liquidto be introduced, is a ratio of the total surface area of all thedroplets, to the number of moles of gas in the chamber. This ratio, insquare meters per mole, could range from about 1 to 250 or more.Examples of this ratio which may be suitable for use in embodiments ofthe present invention include 1, 2, 5, 10, 15, 25, 30, 50, 100, 125,150, 200, or 250.

Embodiments of spray nozzles according to the present invention mayexhibit particular performance characteristics. Examples of performancecharacteristics include breakup length, spray pattern, spray cone angle,fan angle, angle to surface (for fan sprays), and droplet spatialdistribution.

One performance characteristic is droplet size. Droplet size may bemeasured using DV50, Sauter mean diameter (also called SMD, D32, d₃₂ orD[3, 2]), or other measures. Embodiments of nozzles according to thepresent invention may produce liquid droplets having SMD's within arange of between about 10-200 um. Examples of droplet sizes produced byembodiments of nozzles according to the present invention include butare not limited to those having a SMD of about 200 microns, 150 microns,100 microns, 50 microns, 25 microns, and 10 microns.

Another performance characteristic of liquid spray nozzles according toembodiments of the present invention, is flow rate. Embodimentsaccording to the present invention may produce a flow rate of betweenabout 20 and 0.01 liters per second. Examples of flow rates ofembodiments of nozzles according to the present invention are 20, 10, 5,2, 1, 0.5, 0.25, 0.1, 0.05, 0.02, and 0.01 liters per second.

Another performance characteristic of liquid spray nozzles according toembodiments of the present invention, is breakup length. Liquid outputby embodiments of nozzles according to the present invention may exhibita breakup length of between about 1-100 mm. Examples of breakup lengthsof sprays of liquid from nozzles according to the present inventioninclude 100, 50, 25, 10, 5, 2, and 1 mm.

Embodiments of nozzles according to the present invention may producedifferent types of spray patterns. Examples of spray patterns which maybe produced by nozzle embodiments according to the present inventioninclude but are not limited to, hollow cone, solid cone, stream, singlefan, and multiple fans.

Embodiments of nozzles according to the present invention may producespray cone angles of between about 20-180 degrees. Examples of suchspray cone angles include but are not limited to 20°, 22.5°, 25°, 30°,45°, 60°, 90°, 120°, 150°, and 180°.

Embodiments of nozzles according to the present invention may producespray fan angles of between about 20-360 degrees. Examples of such fanangles include but are not limited to 20°, 22.5°, 25°, 30°, 45°, 60°,90°, 120°, 150°, 180°, 225°, 270°, 300°, 330°, or 360°. Examples of fanspray angles to surface possibly produced by embodiments of the presentinvention, include but are not limited to 90°, 80°, 60°, 45°, 30°,22.5°, 20°, 15°, 10°, 5°, or 0°.

Droplet spatial distribution represents another performancecharacteristic of liquid spray nozzles according to embodiments of thepresent invention. One way to measure droplet spatial distribution is tomeasure the angle of a sheet or cone cross-section that includes most ofthe droplets that deviate from the sheet. In nozzle designs according toembodiments of the present invention, this angle may be between 0-90degrees. Examples of such angles possibly produced by embodiments of thepresent invention include but are not limited to 0°, 1°, 2°, 5°, 7.5°,10°, 15°, 20°, 25°, 30°, 45°, 60°, 75°, or 90°.

Certain nozzle designs may facilitate the fabrication of individualnozzles. Certain nozzle designs may also permit the placement of aplurality of nozzles in a given surface proximate to one another, whichcan enhance performance.

In particular embodiments, sprays of liquid from two or even more of thenozzles may overlap with each other in certain regions. This overlapcreates the potential that the liquid spray droplets will collide witheach other, thereby further breaking them up into smaller sizes for heatexchange.

Nozzles may be positioned on one or more surfaces within a cylinder.Nozzles may be positioned to inject liquid in directions substantiallyparallel to, or orthogonal to, directions of motion of a moveable memberwithin a chamber, and/or directions of gas inlet into a chamber.

The flexibility in fabrication and placement of a plurality of spraynozzles, may offer additional enhancements to performance. For example,in certain embodiments the orientation of the dimensional axis of spraystructures relative to a direction of piston movement and/or a directionof gas inflow, may be uniform or non-uniform relative to other spraystructures.

Thus in certain embodiments, the dimensional axis of the spraystructures could each be offset from a gas flow direction in aconsistent manner, such that they combine to give rise to a bulk effectsuch as swirling. In other embodiments, the dimensional axis of thespray structures could be oriented in a non-uniform relative to certaindirection, in a manner that is calculated to promote interaction betweenthe gas and the liquid droplets. Such interaction could enhancehomogeneity of the resulting mixture, and the resulting properties ofthe heat exchange between the gas and liquid of the mixture.

In certain embodiments, one or more spray nozzles may be intentionallyoriented to direct a portion of the spray to impinge against the chamberwall. Such impingement may serve to additionally break up the spray intosmaller droplets over a short distance.

While the '223 Publication has focused upon the use of compressed gasfor energy storage applications, embodiments are not limited to thisparticular application, and could be applied to other fields involvingthe storage of compressed gases.

For instance, compressed air may be utilized as an energy source for avariety of different industrial applications. Examples of suchapplications include but are not limited to painting, combustion (e.g.turbines, blast furnaces), and powering pneumatically-driven tools suchas drills and hammers.

In such applications, the compressed gas storage unit may be configuredto perform certain additional functions, such as conditioning the gasprior to, during, or subsequent to its storage. For example, accordingto certain embodiments compressed air may be dried prior to itsdeployment in a pneumatic system.

In another example, compressed gas may be partially expanded from a highstorage pressure in order to render it useful for certain applications.Thus while air may be compressed to a pressure of 3000 psig for storagepurposes, prior to being used to operate pneumatic tools it may need tobe expanded to reach a lower pressure (e.g. a shop pressure of 500psig).

FIG. 30 shows an embodiment of a system that may be suited for such apneumatic application. The system 3000 comprises multiple gas storageunits 3002 and 3004: High pressure gas storage unit 3002 is configuredto receive and efficiently store gas at high pressure (P_(St)) as aresult of serial compression by a low pressure stage 3005 and a highpressure stage 3006.

Low pressure gas storage unit 3004 is configured to receive gas at alower pressure (P_(Work)). Such gas may be partially expanded from thehigh pressure stage 3002, or gas that has been compressed only by thelow pressure stage 3005. The low pressure gas may be flowed via valve3010 through conduit 3011 to a pneumatic system (e.g. shop air system)in order to power various functionalities (e.g. painting, drilling,etc.). Where the low pressure gas is obtained by partial expansion in ahigh pressure stage (rather than by only partial compression by a lowerpressure stage), that partial expansion can provide desired electricalpower and/or cooling functions.

Still another gas conditioning function which can be integrated with astorage unit according to various embodiments, is gas-liquid separation.Such separation may be particularly useful where the gas has beencompressed in the presence of liquid as a heat exchange medium, forexample as part of a near-isothermal compression process. Other gasconditioning functions that can be implemented in a storage unitstructure can include gas filtering and/or thermal conditioning. Inconnection with the latter, heat exchangers may be incorporated into thegas storage unit.

Yet another function which can be integrated into a storage unitaccording to various embodiments, is pressure stabilization. Inparticular, the compressed gas storage unit features a structure toensure substantially constant internal pressure.

According to some embodiments, this structure may comprise a solidpartition such as a wall or a flexible diaphragm (e.g. bladder), that ismoveable within the storage unit in order to adjust its effective volumeavailable to receive compressed gas. Alternatively, this structure maycomprise liquid moveable into and out of the storage unit, again toadjust the effective volume that is available to receive and hold thecompressed gas.

What is claimed is:
 1. A compressed gas storage unit comprising: a firstshell formed by blow molding a first polymer parison within a mold; anda second shell disposed within the first shell and formed by blowmolding a second polymer parison.
 2. A compressed gas storage unit as inclaim 1 wherein the second shell is formed by blow molding the secondpolymer parison within the first shell.
 3. A compressed gas storage unitas in claim 1 wherein the second shell is inserted into the first shell.4. A compressed gas storage unit apparatus as in claim 1 wherein thesecond shell is in contact with the first shell.
 5. A compressed gasstorage unit apparatus as in claim 1 further comprising an interveningmaterial between the second shell and the first shell.
 6. A compressedgas storage unit apparatus as in claim 5 wherein the interveningmaterial comprises an epoxy.
 7. A compressed gas storage unit apparatusas in claim 1 wherein the first shell further comprises a flangefeature.
 8. A compressed gas storage unit apparatus as in claim 1wherein the first shell comprises ultra high molecular weightpolyethylene (UHMWPE).
 9. A compressed gas storage unit apparatus as inclaim 1 wherein the first shell comprises polyethylene terephthalate(PET).
 10. A compressed gas storage unit apparatus as in claim 1 whereinthe first shell has a shape comprising a cylinder, a sphere, ahemisphere, or a cone.
 11. A compressed gas storage unit apparatus as inclaim 1 further comprising a wound filament in contact with the firstshell.
 12. A method of fabricating a compressed gas storage unit, themethod comprising: disposing a first polymer parison within a molddefining a cavity; blow molding the first polymer parison within themold to create a first shell conforming to a shape of the cavity; andproviding a second shell within the first shell.
 13. A method as inclaim 12 wherein providing the second shell comprises: disposing asecond polymer parison within the first shell; and blow molding thesecond polymer parison within the first shell to define the secondshell.
 14. A method as in claim 12 wherein providing the second shellcomprises: blow molding the second shell from a second polymer parisonwithin a second mold; and inserting the second shell within the firstshell.
 15. A method as in claim 12 further comprising introducing amaterial within the first shell prior to providing the second shell. 16.A method as in claim 15 wherein the material comprises an epoxy.
 17. Amethod as in claim 12 wherein the shape of the cavity is cylindrical.18. A method as in claim 12 wherein the shape of the cavity isspherical.
 19. A method as in claim 12 wherein the first polymer parisoncomprises ultra high molecular weight polyethylene (UHMWPE) orpolyethylene terephthalate (PET).
 20. A method as in claim 12 whereinthe first shell comprises polyethylene terephthalate (PET).
 21. A methodas in claim 12 further comprising winding a filament around the firstshell.
 22. A compressed gas storage unit comprising: a molded plasticliner defining an internal chamber; and a wound filament in contact withthe molded plastic liner.
 23. A compressed gas storage unit as in claim22 wherein the molded plastic liner comprises a blow molded shell.
 24. Acompressed gas storage unit as in claim 22 wherein the molded plasticliner is molded within the wound filament.
 25. A compressed gas storageunit as in claim 22 wherein the molded plastic liner is rotary molded.26. A compressed gas storage unit as in claim 22 wherein the moldedplastic liner includes a safety feature to ensure that the liner willleak at a pressure lower than a burst pressure of the wound filament.27. A compressed gas storage unit as in claim 22 wherein the moldedplastic liner comprises two or more pieces connected by a jointconfigured to leak at a pressure lower than a burst pressure of thewound filament.
 28. An apparatus comprising: a compressed gas storageunit defining an internal chamber; and a heat exchanger disposed in theinternal chamber to communicate heat to and from stored compressed gas.29. An apparatus as in claim 28 wherein the heat exchanger comprises acirculating heat exchange fluid.
 30. An apparatus as in claim 29 whereinthe heat exchanger comprises a heat pipe.
 31. An apparatus as in claim29 wherein the circulating heat exchange fluid is thermal communicationwith a heat source and/or heat sink through an external heat exchanger.32. An apparatus as in claim 29 wherein the circulating heat exchangefluid is actively circulated.
 33. An apparatus as in claim 29 whereinthe circulating heat exchange fluid comprises a liquid.
 34. An apparatusas in claim 29 wherein the heat exchanger comprises a tube configured tocirculate the liquid heat exchange fluid.
 35. An apparatus as in claim28 wherein the heat exchanger comprises a finned tube providinglongitudinal support to the molded plastic liner.
 36. An apparatus as inclaim 28 wherein the heat exchanger is configured to introduce the heatexchange fluid into the chamber, and the compressed gas storage unitfurther comprises a drain.
 37. A compressed gas storage modulecomprising a plurality of compressed gas storage units in selectivefluid communication, each compressed gas storage unit having a dimensionto allow the compressed gas storage module to conform to a standard formfactor.
 38. A compressed gas storage module as in claim 37 wherein thestandard form factor allows the compressed gas storage module to betransported by road.
 39. A compressed gas storage module as in claim 37wherein the standard form factor allow the compressed gas storage moduleto be certified for overseas shipping.
 40. A compressed gas storagemodule as in claim 37 wherein the plurality of compressed gas storageunits is fabricated by blow molding.
 41. A compressed gas storage moduleas in claim 37 wherein the plurality of compressed gas storage units isfabricated by filament winding.
 42. A compressed gas storage module asin claim 37 wherein the plurality of compressed gas storage units areconfigured to be in selective fluid communication with an energy storagedevice, and are sized to provide a standard capacity of output power.